1. Field of the Invention
The present invention relates to an ultrasonic bone evaluation apparatus for evaluating bone of a living organism by using ultrasound.
2. Description of the Related Art
Osteoporosis is of great concern in recent years that provokes an increasing need for evaluation of bone. While a mainstream method conventionally used for bone assessment is radiological diagnosis using X rays, ultrasonographic bone assessment is becoming increasingly popular today. Compared to the conventional X-ray method, ultrasonography is advantageous in that an ultrasonic diagnostic apparatus can be constructed more compactly and relieve stress on examinees requiring less measurement time and no exposure to X rays. An index often used for bone assessment is the speed of sound (SOS) at which ultrasound waves propagate through bone tissue.
Japanese Patent No. 2840040 discloses one conventional approach to measuring the speed of sound propagating through a body part of an examinee with high accuracy. This approach involves the use of a temperature sensor for measuring the temperature of an acoustic matching medium filled within a transducer cover. The speed of sound passing through the acoustic matching medium is corrected based on the temperature thereof measured by the temperature sensor to obtain an accurate measurement of the ultrasound speed through the body part being examined.
Another previous approach is found in Japanese Patent Application Publication No. 1994-197895. This approach makes use of a vessel containing acoustic matching liquid in which a body part to be examined is placed and a temperature sensor for measuring the temperature of the body part. The temperature of the acoustic matching liquid is adjusted based on the measured temperature of the body part, and measurement of ultrasonic wave propagation property (speed of sound) is commenced when the temperature of the acoustic matching liquid and the temperature of the body part reach equilibrium with each other. According to the Publication, this approach makes it possible to perform ultrasonographic measurement with good repeatability in a stable fashion.
The calcaneus is most often used in the evaluation of bone architecture by an ultrasonographic measuring apparatus. Generally, the speed of ultrasound propagating through the calcaneus measured by the apparatus is affected by the temperature of the heel which causes considerable variations in ultrasound speed measurements. Such variations in ultrasound speed measurements are particularly significant during winter when the temperature of the heel is low.
Conventionally, an examinee is forced to wait in a well-heated waiting room for a long time (nearly one hour) before measurement until the heel warms. Although measurement errors can be almost avoided in this way, this conventional approach is not quite efficient because the examinee must spend a long waiting time, resulting in a reduction in measurement efficiency.
The aforementioned approach of Japanese Patent No. 2840040 is to simply correct the speed of sound passing through the acoustic matching medium by using the temperature of the acoustic matching medium measured by the temperature sensor, so that variations in the ultrasound speed through the heel due to changes in the temperature of the heel can not be corrected by this approach.
According to the aforementioned approach of Japanese Patent Application Publication No. 1994-197895, on the other hand, measurement can be started only after the temperature of the body part to be examined has become equal to the temperature of the acoustic matching liquid. Thus, this approach requires a long measurement time and results in poor measurement efficiency. In addition, the examinee must place the body part to be examined (typically, one of the feet) in the acoustic matching liquid filled in the vessel with the approach of this Publication. This is not quite comfortable for the examinee and makes it necessary to take measures to maintain properly controlled hygienic conditions.
There exists another conventional approach to reducing variations in the ultrasound speed through the heel due to changes in the temperature of the heel. This approach is to combine the aforementioned measurement of the speed of sound (SOS) and a measurement of frequency-dependent broadband ultrasonic attenuation (BUA) of the body part being measured. This approach requires complex calculation and imposes heavy load on a processing device which performs mathematical operation for bone assessment.